The present invention relates to a method of measuring a furnace temperature in a hot isostatic pressing (HIP) unit and a device for measuring same, and more particularly to a method of measuring the furnace temperature in the HIP unit including an improved temperature measuring optical system for collecting thermal radiation from an end portion of a closed-end tube. Further, the present invention relates to a method of measuring the furnace temperature in the HIP unit which is improved in a wavelength range to be used for sensing of the temperature.
The HIP unit is used for carrying out pressure sintering of powder, removal of defects in a sintered product or a forged product, or diffused junction owing to a synergetic effect by high temperature and high pressure. In recent years, industrial utilization of the HIP unit has become noticeable, and application thereof has been recently expanded to a high temperature range of 1700.degree.-2100.degree. C. for an engineering ceramics.
In the HIP unit, temperature control of a furnace at high temperature under high pressure is greatly significant for treatment effect, and there have been proposed various temperature measuring means for detecting the furnace temperature. At present, radiation temperature measuring means utilizing a closed-end tube has been adopted, for example.
FIGS. 9 and 12 show some examples of conventional HIP units including such furnace temperature measuring means.
The HIP unit as shown in FIG. 9 includes a closed-end tube 15 and an optical system 16. A sample bed 14 is placed on a lower cover 13 of a high pressure vessel 11 containing a heat insulating layer 12. The interior of the vessel 11 is partitioned by the heat insulating layer 12 to define a furnace chamber in which the closed-end tube 15 is located in such a manner that an end portion of the closed-end tube 15 is exposed to an area to be detected. Thermal radiation from the closed-end tube 15 is introduced to the outside of the furnace chamber via the optical fiber 16 connected at its one end to a lower end of the closed-end tube 15. The optical fiber 16 is connected at the other end to a radiation thermometer 17 constituting a measuring system. This unit is disclosed in Japanese Patent Laid-Open No. 60-133327. As shown in FIG. 10, the radiation beam from the closed-end tube 15 is allowed to directly enter the optical fiber 16. Alternately, as shown in FIG. 11, the radiation beam is condensed by a collimator 20 using a lens 19 to enter the optical fiber 16.
In the HIP unit as shown in FIG. 12, long and short slender cylindrical tubes 30 and 31 closed at their upper ends and open at their lower ends are located in such a manner that respective upper portions thereof are enclosed in a furnace chamber or a treating chamber, while the lower ends are positioned outside the treating chamber. Measuring terminals 32 and 33 of a radiation thermometer are mounted at the open lower ends of the tubes 30 and 31 so that a focal point may be formed at the close upper ends of the tubes 30 and 31. Detection signals from the measuring terminals 32 and 33 are fed through optical signal cables 34 and 35 to a photoelectric converter 36 in the HIP unit, thereby taking out an output corresponding to the temperature through lead wires 37 passing through a high pressure vessel. The output is supplied to a treating chamber temperature automatic control device 38 and a thyristor control device 39 to control upper and lower heaters 40 and 41 through lead lines 25. This unit is disclosed in Japanese Patent Laid-Open No. 60-144627.
However, the place where the optical system is located is usually subjected to a temperature of 300.degree. C. under a pressure of 2000 atm, and a density of Ar or N.sub.2 gas forming such an atmosphere is higher than that at ordinary temperature and ordinary pressure. Especially, as temperature of a part where the collimator (FIG. 11) is located in the unit shown in FIG. 9 is relatively low, an atmosphere around the part has more increased density.
As a result, a refractive index of the gas increases with an increase in the density, and becomes higher than a value at ordinary temperature and ordinary pressure. Accordingly, optical characteristics of the lens and the optical fiber, which are designed for use in the air under the conditions of ordinary temperature and ordinary pressure, e.g., a focal distance of the lens and a numerical aperture of the optical fiber are changed to cause an influence upon thermometer characteristics.
Describing the above in detail, the focal distance of the lens is usually expressed in the following equation. ##EQU1##
Where, r.sub.1 and r.sub.2 are radii of curvature of both surfaces of the lens; n=n.sub.L /n.sub.g, where n.sub.L is an absolute refractive index of the lens, and n.sub.g is an absolute refractive index of a medium around the lens.
When the medium gas is in the conditions of ordinary temperature and ordinary temperature, n.sub.g is substantially equal to 1, and the lens is designed under such conditions.
However, the absolute refractive index of the gas changes with pressure as shown in the following table, and therefore, the focal distance is changed according to the above-mentioned equation. (High-Pressure Testing Techniques and Their Applications, pp. 441)
TABLE ______________________________________ N.sub.2 Ar ______________________________________ Temp.: 25.degree. C. Temp.: 25.degree. C. Wavelength: 5876A Wavelength: 5876A ______________________________________ Refractive Refractive Pressure (atm) Index Pressure (atm) Index ______________________________________ 1 1.0002728 1 1.0002580 61.164 1.01665 21.4042 1.00576 162.332 1.04333 35.0086 1.00921 162.455 1.04332 50.6661 1.01365 304.27 1.07311 69.8965 1.01878 405.76 1.08911 100.809 1.02761 528.50 1.10429 201.127 1.05522 750.74 1.12432 381.490 1.09522 894.59 1.13414 690.792 1.13505 1138.12 1.14752 1040.37 1.16078 1646.63 1.16763 1539.35 1.18409 2053.07 1.17941 2369.75 1.20899 ______________________________________
Although the density tends to decrease naturally because of high temperature as well as high pressure in the HIP unit, and a change rate of the refractive index is less than that shown in Table, the condition of the temperature measuring optical system is yet changed.
In the circumstances, the conventional temperature measuring means does not satisfactorily follow the change in the refractive index of the medium gas due to operational conditions of the HIP unit, and cannot attain stable measurement of temperature.
Furthermore, in the method where the radiation beam from the end portion of the closed-end tube is allowed to directly enter the optical fiber, thermal radiation from a side wall of the closed-end tube 15 having a temperature distribution is also allowed to enter the optical fiber because of a wide angle of visible field of the optical fiber, and such thermal radiation is added to the thermal radiation from the end portion of the closed-end tube, thereby causing an error of temperature measurement at the end portion.
To eliminate this error, there has been conducted a test to find that a temperature measurement error at temperatures near 2000.degree. C. can be suppressed to 1% or less by setting a detection wavelength to 0.6 .mu.m or less. However, in the case that the collimator is used to limit the field of observation at the end portion of the closed-end tube, the above-mentioned setting of the wavelength is not required.
Although the temperature measurement error is reduced by shortening the wavelength, a limited short wavelength is to be considered from limitation of optical materials, and it has been found that detection of the radiation beam is difficult by the wavelength of 0.2 .mu.m or less and that the wavelength equal to or more than 0.3 .mu.m is preferable.
Additionally, provided that a photon counting method by means of a photomultiplier (PM) is employed, dependency of temperature resolution upon wavelength has been calculated for an object at 1000.degree. C. with a time constant of one second. As the result of calculation, it has been clarified that the wavelength equal to or more than 0.3 .mu.m is required for 1 K or more of temperature resolution.
Accordingly, the temperature measurement by the radiation thermometer is actually conducted within the wavelength range of 0.3-0.6 .mu.m, while it is conducted within the range of 0.3 .mu.m or more in the case of limiting a field of observation at a measurement objective point by means of a collimator or the like.
However, as to spectral characteristics of the radiation beam utilized for the temperature measurement, there is no mention except a black body and a gray body.
An actual spectrum of the radiation beam from the HIP unit is shown in FIG. 8, wherein a clear absorption is observed. This absorption is caused by vaporization of a low b.p. metal from a treatment material in the furnace or a capsule glass in the HIP unit. That is, the low b.p. metal in the high-temperature closed vessel of the HIP unit is vaporized during an increase in temperature to generate a gas, which stays in the vessel without being discharged therefrom. As a result, there occurs absorption by the metal gas existing in a light path between the objective point and the collimator in conducting a radiation temperature measurement.
However, the conventional temperature measuring method does not consider such absorption by the metal gas, and there is a possibility that the absorption by the metal gas is overlapped on the wavelength to be used for the temperature measurement to cause a large error.